Deformation of the Earth's crust by glacial loading and unloading was first recognized more than a century ago (De Geer 1888). This deformation is expressed by postglacial isostatic rebound and is ongoing in large parts of glaciated regions. It is typically a process with uplift rates of millimetres per year (Johansson et al. 2002). The crustal rebound which initially followed deglaciation was, however, faster and therefore caused large earthquakes under certain conditions (Lundqvist & Lagerbäck 1976; Steffen et al. 2014). In Lake Vättern, southern Sweden, mass wasting and deformation structures have been mapped in the sediment accumulated on the lake floor (Jakobsson et al. 2014). Along an 80 km stretch of the lake, deformation structures seen in sediments reveal up to 13 m of vertical displacement in the underlying bedrock. From this displacement, an earthquake with seismic magnitude up to 7.5 was interpreted to have occurred in Lake Vättern shortly after the Scandinavian Ice Sheet had retreated from the area (Jakobsson et al. 2014).
Description
Lake Vättern is a 135 km long and 31 km wide lake in south–central Sweden, located in a SSW–NNE-trending graben (Fig. 1a, b). It is the second largest lake in Sweden. The graben is aligned closely with a zone of bedrock deformation along which two Precambrian bedrock provinces meet (Bingen et al. 2008). Rifting at about 700–800 Ma ago is suggested to have formed the graben now hosting Lake Vättern basin (Andréasson & Rodhe 1990). An outlet glacier of the Scandinavian Ice Sheet filled the basin during the last deglaciation, terminating in a proglacially dammed lake which accumulated a thick sequence of glacilacustrine clays.
(a–d) Swath bathymetry and (e) sub-bottom profile from Lake Vättern. (a) Surveyed areas of southern Lake Vättern and location of profile (e). Bathymetric contours are from Norrman (1964). Multibeam acquisition system Kongsberg EM2040 and EM3000D. Frequency 300 kHz. Grid-cell size 2–5 m. Sub-bottom acquisition system Edgetech chirp SB216, 20 ms pulse length. Frequency 2–12 kHz. VE×8 (profile (e)). (b) Location of study area (red box; map from GEBCO_8.0).
Multibeam swath-bathymetric mapping and sub-bottom profiling of the southern, deepest section of Lake Vättern collected during field work by Stockholm University in 2008 and 2013, revealed narrow depressions in the lake bottom (Jakobsson et al. 2014). These depressions form linear arrays that are parallel to the SSW–NNE elongation of the lake basin. They range from a few metres to >10 m depth and are up to 100 m wide (Fig. 1c, d). The largest depression is estimated to be 13 m deep. The depression structures are seen clearly in sub-bottom profiles collected perpendicular to the long axis of the lake (Fig. 1e). Their morphology suggests that they are deformation structures formed from the downward collapse of the sediment stratigraphy. Close inspection of several sub-bottom profiles reveals that there is a drape of sediment on top of the collapse structures, suggesting that they are relatively old features and have not formed from an ongoing subsidence process.
The eastern slope of Visingsö Island reaches an inclination of about 20°. Three large slide scars and associated mass-wasted material were mapped on this slope (Fig. 1c, d). The southernmost and largest of these slides extends over a distance of 1700 m (Jakobsson et al. 2014). The northernmost part of the largest slide and the other two slides are imaged in Figure 1d.
Interpretation
The mapped depressions in the soft bottom sediment of Lake Vättern were interpreted by Jakobsson et al. (2014) as collapse structures formed by tectonic movements of the underlying bedrock during an earthquake. Empirical relationships derived by Wells & Coppersmith (1994) between seismic-moment magnitude, maximum vertical displacement and maximum surface rupture length along a fault were used by Jakobsson et al. (2014) to estimate that the earthquake reached a seismic-moment magnitude of 7.5. This estimate assumes that the mapped structures were formed during one large seismic event rather than during several smaller consecutive events.
A sediment core, retrieved east of Visingsö Island, captured the contact between the overlying sediment drape and the underlying sediment sequence containing the collapse structures (Jakobsson et al. 2014). Dating of the sediment core suggests that the major earthquake occurred at the Younger Dryas–Preboreal transition, c. 11.5 ka BP. This timing places the major earthquake a few hundred years after the Scandinavian Ice Sheet left the northernmost part of Lake Vättern and after the sudden drainage of the Baltic Ice Lake, which occurred at the very end of Younger Dryas at c. 11.7 ka BP (Björck 1995). It has not been possible to date the large landslides, although their locations close to the collapse structures led Jakobsson et al. (2014) to suggest that they may have been triggered by the large earthquake.
- © 2016 The Author(s). Published by The Geological Society of London. All rights reserved
References
Postglacial tectonic structures and mass wasting in Lake Vättern, southern Sweden
